Consider adding something for pileup IPs to RapidParticle to avoid current caching hack

[dcraik]

RapidDecay::genDecay currently has to cache IPs from different PVs due to how the IP smearing is set up.

One option around this might be to add a function to RapidParticle that calculates IP etc for a passed PV and internally tracks the minimum IP as well as the "ownPV" one.